Perfluorosulfonic acid membranes typified by DuPont Nafion (registered trademark) have been used widely as electrolyte membranes for polymer electrolyte fuel cells. However, these perfluorosulfonic acid membranes present problems such as low proton conductivity at insufficient humidity, low power generation efficiency attributed to the easy permeation of liquid fuels used, such as alcohols, and a large dimensional change during swelling.
To solve these problems, studies have been made on an electrolyte membrane obtained by filling a proton conducting polymer into holes of an insulating porous film. This electrolyte membrane, which uses the insulating porous film as a base material, has advantages that the electrolyte membrane can suppress a dimensional change attributed to moisture and can suppress alcohol permeability even in a moisture state.
As an example of such a polymer electrolyte membrane, Japanese Patent Application Laid-Open No. 2003-263998 discloses an electrolyte membrane obtained by filling 2-acrylamido-2-methylpropanesulfonic acid and a crosslinking agent into a polyimide porous film and performing polymerization.
Alternatively, as another example of a polymer electrolyte membrane, Japanese Patent Application Laid-Open No. 2004-253336 discloses an electrolyte membrane obtained by filling 2-methacrylamido-2-methylpropanesulfonic acid and a crosslinking agent into a crosslinked polyethylene film and performing polymerization.
However, the polymer electrolyte membranes prepared by the production methods described in Japanese Patent Application Laid-Open Nos. 2003-263998 and 2004-253336 are considered to have insufficient interface contact between the lipophilic porous polymer film and the hydrophilic proton conducting component (proton conducting filling). Therefore, the proton conducting component is not sufficiently fixed in holes of the polymer film even after polymerization. Thus, it is expected that satisfactory output characteristics are not obtained due to the increased internal resistance of a cell and a reduced output in a continuous run.
Alternatively, Japanese Patent Application Laid-Open No. 2005-120198 discloses, as an independent membrane, an electrolyte membrane including a bicyclo compound. This document discloses dicyclopentanyl and dicyclopentenyl as examples of the bicyclo compound.
However, the polymer electrolyte membrane described in Japanese Patent Application Laid-Open No. 2005-120198 is an independent membrane. Therefore, this electrolyte membrane probably presents problems such as a large dimensional change during swelling.
Thus, the present invention improves the interface contact between a porous polymer film and a proton conducting component and enhances the fixation efficiency of a filling. As a result, the present invention provides a polymer electrolyte membrane having power generation characteristics with a high output and long life, a method for producing the same, a membrane electrode assembly, and a polymer electrolyte fuel cell.